{
  "circularId": 43050,
  "subject": "GRB 251208B: Swift-XRT afterglow detection",
  "body": "J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester),\nJ.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),\nA. D'Ai (INAF-IASFPA), S. Lanava (PSU) and P.A. Evans (U. Leicester)\nreports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the GUANO (DeLaunay\net al. GCN 43033) and Fermi/LAT-detected (Bissaldi et al. GCN 43034)\nburst GRB 251208B. We searched for X-ray sources in  4.8 ks of Photon\nCounting (PC) mode data. The total exposure at the position of the\nafterglow (see below) is 4.8 ks, obtained between T0+33.1 ks and\nT0+45.4 ks.\n\nThree uncatalogued X-ray sources are detected within the estimated\n3-sigma Fermi/LAT error region (414 arcsec), of which one (\"Source 1\")\nis 2.9 sigma above the RASS limit and it is fading with 2.2 sigma\nsignificance and thus is believed to be the GRB afterglow. Using 4576 s\nof PC mode data and 5 UVOT images, we find an enhanced XRT position\n(using the XRT-UVOT alignment and matching UVOT field sources to the\nUSNO-B1 catalogue): RA, Dec = 228.48108, +29.04575 which is equivalent\nto:\n\nRA (J2000): 15h 13m 55.46s\nDec(J2000): +29d 02' 44.7\"\n\nwith an uncertainty of 2.3 arcsec (radius, 90% confidence). This\nposition is 87 arcsec from the Fermi/LAT position.  The light curve is\nconsistent with a constant source of mean count rate 5.0e-02 ct/sec. A\npower-law fit gives an index of 1.2 (+/-1.2).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.2 (+0.4, -0.3). The\nbest-fitting absorption column is  1.4 (+1.1, -0.8) x 10^21 cm^-2, in\nexcess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 3.1 x 10^-11 (4.4 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.4 (+1.1, -0.8) x 10^21 cm^-2\nGalactic foreground: 2.6 x 10^20 cm^-2\nExcess significance: 2.3 sigma\nPhoton index:\t     2.2 (+0.4, -0.3)\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/00021893.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021893.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "eventId": "GRB 251208B",
  "bibcode": "2025GCN.43050....1K",
  "createdOn": 1765288389951,
  "submittedHow": "email",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>"
}